Some low-impact improvements to composer

I've split my original pull request #599 into a number of separate requests. This first one consists of low-impact changes, in that there's no change to UI and no extra strings added.

This one contains:
- shift clicking an already selected item deselects the item
- holding shift while pressing cursor keys results in a large movement
- panning in composers using the middle mouse button
- zoom composers using the mouse wheel

Not sure if these can be accepted into 2.0, if not I'll hold off till 2.1.